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Annual State of the Climate Report for 2015 now published

August 2016 - Last year (2015) was yet another notable year for global climate. It was the warmest year on record by a large margin and another year in a now long string of years when the levels of dominant greenhouse gases reached new peaks.

For the second consecutive year, global temperatures hit record levels, surpassing the record set in 2014 and also reaching 1°C above pre-industrial* conditions for the first time. This is accompanied by a record increase in CO2 and record highs in all of the major greenhouse gases, ocean heat content, sea level, frequency of warm days, total column water vapour over ocean and specific humidity over ocean.

All of these data are presented in the 'State of the Climate in 2015' report, the 26th consecutive instalment to the annual reports produced by theAmerican Meteorological Society. These comprehensive reports are led by scientists from the NOAA National Centers for Environmental Information, along with hundreds of scientists from all over the world. Met Office scientist Kate Willett leads the Global Climate chapter with Robert Dunn as a co-editor, and several other Met Office scientists contribute. The report provides a detailed update on global and regional Essential Climate Variables (ECVs) using all available climate monitoring records, including several produced by the Met Office Hadley Centre's HadOBS data portal. Patterns, trends and changes are tracked across the global climate system. This year's report and all previous editions are freely available online.

Temperature

The year 2015 saw exceptional warmth, exceeding the record year of 2014 by more than 0.1°C and temperatures representative of the pre-industrial period by 1°C for the first time. Much of the warmth was over the oceans, particularly the El Niño region of the tropical Pacific. Ocean heat content and sea level also reached record highs. Over land surfaces, despite record high warm day frequency and record low cool day frequency, regional warmth did not surpass the record levels of 2014 with the exception of Canada. However, record or near-record warmth was reported in at least one location across every inhabited continent.

Modes of Variability

The presence of a strong El Niño during 2015 was a driver of many of the features seen throughout the ocean and atmosphere. By some measures, the El Niño began in 2014, and indeed features common to such a protracted event were apparent with persistent drought over Queensland, Australia. By other measures the El Niño began in earnest in 2015. Regardless, the event was comparable with other strong events of 1982/83 and 1997/98 in its severity and impact. On the global scale, El Niño contributed to generally drier-than-average conditions including an increase in global land area experiencing severe drought.

Snow and Ice

In the cryosphere (regions of the world containing snow and ice) the effect of increased warming continued to be visible in the decline of glacier volume. With the addition of four more reference glaciers compared to the 2014 report, preliminary results for 2015 make it the 36th consecutive year of negative glacier mass balance (i.e., an overall decline in glacier mass). Northern Hemisphere snow cover was below its 46 year average. Arctic sea ice was well below average but above the exceptional lows seen in 2007, 2011 and 2012. However, 2015 saw the lowest maximum extent of Arctic sea ice in its 37 year record.

Water

The strong El Niño drove distinct patterns across the hydrological cycle. Soil moisture was below average for the entire year and terrestrial groundwater storage reached a record low. The area of land across the globe experiencing 'severe' drought increased considerably from 8% at the end of 2014 to 14% by the end of 2015. Conversely, atmospheric moisture levels were high, especially over oceans, although land surface relative humidity remained well below average.

Atmospheric Chemical Composition

The atmospheric concentration of long-lived greenhouse gases (CO2, CH4 and N2O) continued to increase, all reaching new record highs and bringing total radiative forcing to 2.98 W m-2, 38% above 1990 values. Atmospheric CO2 concentrations reached 399.4 ppm for the global average. At the Mauna Loa Observatory, Hawaii, the longest CO2 record, the 2015 annual average exceeded 400 ppm for the first time in its 56 year record. This represented the largest annual increase on record of 3.05 ppm. Large increases are common in strong El Niño years but 2015 surpasses that of the strong 1997/98 El Niño.

Stratospheric ozone is very variable year-to-year. Although Antarctic ozone reached a near-record low September to December, annual mean stratospheric ozone levels are showing small signs of recovery in response to the Montreal Protocol.

The dry conditions exacerbated by the strong El Niño led to severe wildfires during 2015. Over tropical Asia, particularly Indonesia, carbonaceous aerosols from biomass burning were almost three times the 2001-2014 average.

Global Interconnectivity

The comprehensive set of observations in this report enables a view of the interconnected nature of the Earth's climate system. After two years of absence an analysis of Lake Temperatures has returned. These show long-term trends in many cases, consistent with many other features of the climate system. Also included within the Global Climate chapter (Chapter 2) are two sidebars. The first discusses the status of monitoring evaporation over land, a crucial missing link in our understanding of the hydrological cycle. The second provides an overview of atmospheric chemical composition in response to the El Niño driven forest fires. As part of Chapter 1 there is a Met Office-led sidebar discussing the 2015/16 El Niño in comparison with other strong El Niños.

Sub notes

* Pre-industrial is taken here to mean the 1850-1900 period as this is the closest instrumental record period that can be considered representative of the pre-industrial period. While late 19th century temperatures are commonly taken to be indicative of pre-industrial, there is no fixed period that is used as standard and a variety of other periods have been used for observational and palaeo datasets. There are limitations in available data in the early instrumental record, making the average temperature in the reference period less certain. There is not a reliable indicator of global temperatures back to 1750, which is the era widely assumed to represent pre-industrial conditions. Therefore 1850-1900 is chosen here as the most reliable reference period, which also corresponds to the period chosen by IPCC to represent a suitable earlier reference period.